Electrical generation of suitable tone signals in electronic musical instruments has been accomplished in the past by many different arrangements. In recent years, the prevalent practice has been to provide oscillators for separately generating tone signals corresponding to the semi-tones of the top octave of the instrument or above, and providing each oscillator with a chain of divide-by-two circuits to produce the corresponding frequencies (lower order harmonics) in lower octaves of the instrument. This approach of course requires individual tuning of twelve oscillators.
Recent developments in integrated circuit technology have made possible the use of a single high frequency oscillator driving parallel divider circuits each providing a suitable divider ratio to produce one of the twelve frequencies of the top octave of the instrument. These divider chains are then followed by divide-by-two circuits as in the previous example to produce the corresponding frequencies in lower octaves of the instrument. Such circuits are advantageous in requiring tuning of but a single oscillator. However, these circuits have presented problems in obtaining accurate division ratios and suitable waveform symmetry that are difficult and costly to solve.
In both of the foregoing systems, the use of conventional divider circuits results in generation of square waves (ie., 50% duty cycle rectangular waves), thus limiting the harmonic structure of the tone signal at the outputs of such generators to the harmonic spectrum of a square wave. Thus, to obtain other desired musical sounds, it is necessary to provide other circuitry, which is often quite cumbersome and expensive, to modify, combine, reshape and/or filter these waveforms as necessary to produce the desired musical sounds. For example, some systems utilize analog waveshaping techniques to produce harmonically rich waveforms, and then selectively filter these waveforms to approximate the desired harmonic structure of the output waveform. Generally speaking, such analog circuits are subject to problems in maintaining accurate and noise free signals during analog signal processing. Moreover, such analog circuits have generally been cumbersome and expensive, due to the great redundancy of circuits generally required to produce the number of voices required even for a relatively simple instrument.